Calcium carbonate, magnesium carbonate and magnesium trisilicate are known gastric antacids (for example, see Remington's Pharmaceutical Sciences, 18th Edtn., 1990, published by Mack Publishing Co., Chapter 39 on "Gastrointestinal Drugs) with relatively fast onset of action and prolonged duration of action. However, as noted on page 779 of the Remington reference, which provides a tabular listing of twenty-six proprietary antacid suspensions, only one indicates a combination of calcium carbonate with a magnesium salt (magnesium carbonate) for the product Marblen (Fleming).
It has been observed that aqueous antacid suspensions of calcium carbonate in combination with magnesium carbonate and/or magnesium trisilicate yield very alkaline compositions with a pH of about 9 or even higher to about 9.9. Such high alkalinity is disadvantageous for comercial purposes, for example, preservatives in the formulation such as parabens will break down and lose efficacy over time at such high pH's.